1. Field of the Invention
The present invention relates to a steel plate reinforcing sheet and, more particularly, to a steel plate reinforcing sheet used to be adhesively bonded to steel plate used in a variety of industrial machines such as transportation machines.
2. Description of the Prior Art
In general, a steel plate for a shell of an automotive vehicle is manufactured in the form of a thin sheet having a thickness generally in the range of 0.6 mm to 0.8 mm, for reduction in weight of a vehicle body. As is known, a steel plate reinforcing sheet is adhesively bonded to the inside of the steel plate, for the reinforcement purpose.
This steel plate reinforcing sheet is, for example, adhesively bonded and thermally foamed by using the heat generated at the time of the electrodeposition coating, to develop its reinforcing property in a fabrication process of the steel plate of the shell of the automotive vehicle. For example, JP Laid-open (Unexamined) Patent Publication No. Hei 7-68695 proposes a steel plate reinforcing sheet which comprises a restricting layer formed of glass fiber cloth and the like, and a foaming resin layer formed of material in which rubber, thermoplastic elastomer, inorganic filler, tackifier, softening agent, cross-linking agent, and foaming agent are mixed.
JP Laid-open (Unexamined) Patent Publication No. 2002-283526 proposes a thermally curable adhesive sheet for reinforcing a thin steel plate having a structure wherein uncured adhesive layers of epoxy resin composition having a adhesiveness at room temperature and containing a thermally decomposable foaming agent are laminated in layers on a base material layer of uncured epoxy resin composition.
Meanwhile, the steel plate for the shell of the automotive vehicle is often designed in a curved form. Due to this, the steel plate reinforcing sheet may also be required to be adhesively bonded to an irregularly curved surface, such as a two-dimensionally or three-dimensionally curved surface. Also, in the winter season, the steel plate reinforcing sheet may be required to be adhesively bonded under a low temperature of around 5° C. If the adhesion of the steel plate reinforcing sheet is insufficient, then the steel plate reinforcing sheet may fall off from the steel plate after it is adhesively bonded to the steel plate and before it is thermally foamed, inducing the disadvantage of reduction in working efficiency.
On the other hand, an attempt to ensure the sufficient adhesion of the steel plate reinforcing sheet requires reduction in viscosity of a bonded surface of the steel plate reinforcing sheet. However, the reduction of viscosity induces the disadvantage that the steel plate reinforcing sheet after thermally foamed may fail to develop its reinforcing property sufficiently.
Also, a reinforcing layer formed of epoxy resin as a base resin, including, for example, the above mentioned thermally curable adhesive sheet for reinforcing the thin steel plate and the like has the problem that it reduces in adhesion particularly to an oiled steel plate in the winter season, so that it may fall off from the steel plate in any of various processes including degreasing, pretreatment, and electrodeposition coating.
On the other hand, softening the base resin to improve the adhesiveness induces reduction in strength of the steel plate reinforcing sheet after cured, leading to failure to provide sufficient reinforcement.